Spectrometers are widely used in many fields such as photometric measurement of a radiation source, beam-splitting, and substance detection. A spectrometer typically includes an entrance slit to control the amount of light entering the spectrometer, and also includes a diffraction grating in combination with a collimation and a correcting lens to focus spectral components on an image plane. A photo detector can be placed on the image plane to detect individual spectral components.
FIG. 1 is a schematic diagram of a conventional spectrometer. Referring to FIG. 1, the conventional spectrometer 5100 includes a light source 5110, an input unit 5120, a collimating lens 5130, a plane grating 5140, a focusing lens 5150, and a linear photo detector 5160. Optical signals 5200 from the light source 5110 first pass through the input unit 5120 and, after being collimated by the collimating lens 5130, the optical signals 5200 reach the plane grating 5140. The macroscopic profile of the diffraction pattern 5142 of the plane grating 5140 is a plane. The diffraction pattern of the conventional plane grating is created by diamond tools.
The diffraction pattern of the conventional plane grating, which is a regular blaze grating (RBG), is shown in FIG. 2. The diffraction pattern has a fixed pitch P1 and a fixed blaze angle (i.e. regular blaze angle) A1. A line connecting two vertices 5144A and 5144B of the diffraction structure 5144 forms a baseline 5144C. A grating normal 5144N is perpendicular to the baseline 5144C, and an effective normal line 5144M is perpendicular to an effective surface 5144D. As incident light LI strikes the effective surface 5144D, reflected light LR (also known as O-order light), 1-order diffracted light LD+1 and LD−1, and light of 2-order or above will be produced (not shown in FIG. 2). Blaze angle A1 is the angle formed between the grating normal 5144N and the effective normal line 5144M, which is equivalent to the angle formed between the baseline 5144C and the effective surface 5144D. In FIG. 2, the blaze angle of each of the diffraction structures 5144 is equal to A1. The diffraction pattern of this type of diffraction gratings is formed by diamond tools. Since every movement of the diamond tool can only produce one diffraction structure, and one diffraction grating normally contains thousands of diffraction structures, this traditional manufacturing method is very time-consuming and expensive. Moreover, since the blade of the diamond tool is fixed, one diamond tool can only form a single type of diffraction structures with a single blaze angle.
FIG. 3 is a schematic diagram of another conventional grating. As shown in FIG. 3, a plane grating 5140′ has a fixed pitch P1 and two blaze angles, A1 and A2. The diffraction pattern of the plane grating 5140′ is also made by diamond tools; however, two types of cutting tools are needed. First, a first type of cutting tool is used to form diffraction structures having a blaze angle A1. Then, the first type of cutting tool is replaced by a second type of cutting tool, and the second type of cutting tool is used to form diffraction structures having a blaze angle A2.
It is worth mentioning that there is always a positioning problem when changing the cutting tools. For example, if there are errors incurred during the positioning of the second cutting tool, the errors will be introduced into the resulting grating. As another example, errors on the starting engraving point of the second cutting tool will also be introduced into the resulting grating. Particularly, since the dimension of a diffraction structure is very small, e.g., only a few microns (um), small errors incurred during the positioning of the second cutting tool will result in a serious problem. For example, discontinuation in the diffraction structures will result in discontinued output optical signals. Errors may even produce serious stray light, thus adversely affecting output signals. Even worse, it may make diffraction gratings no longer useful.
Accordingly, when using the traditional diamond tool to make a diffraction grating with various blaze angles, the positioning problem will arise at each time the cutting tool is being changed, thus increasing errors in the resulting diffraction grating.